Method and apparatus for manufacturing aluminum die-cast product

ABSTRACT

A method and an apparatus for manufacturing an aluminum die-cast product are provided, wherein an occurrence of shrinkage cavity is eliminated or reduced to a very small amount, and a sound and high-quality aluminum die-cast product having no casting defect can be produced. The method for manufacturing an aluminum die-cast product by performing die casting while a part, in which a casting defect tends to occur due to solidification shrinkage of a melt, which has been pressure-filled in a mold, during a solidification process, is pressurized with a local pressurization pin includes the steps of allowing the local pressurization pin to follow the movement of a solidification interface based on a solidification interface movement speed, which is calculated in advance, of the melt and keeping pushing the local pressurization pin in until the solidification is completed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus formanufacturing an aluminum die-cast product suitable for producingpressure vessels and the like required to have airtightness.

This application is based on Japanese Patent Application No.2006-236216, the content of which is incorporated herein by reference.

2. Description of Related Art

In general, with respect to an aluminum die-cast product, it is knownthat a solidification shrinkage space centering a portion, in which thesolidification speed of a melt filled in a mold is the smallest, occursresulting from the solidification shrinkage during die casting. In analuminum alloy, about 6% of volume shrinkage occurs duringsolidification. When the resulting shrinkage space is included in theinside of a product, a shrinkage cavity occurs in a concentrated manner,and this results in a casting defect. The shrinkage cavity tends toextend over a wide range as the thickness of the casting portion isincreased and the mold temperature is increased (temperature gradient issmall).

When the product is a pressure vessel, the above-described shrinkagecavity, which is a casting defect, causes pressure leakage, and is oneof an important factors leading to reduction of the yield of product.

A local pressurization method of pressurizing partly a part in which ashrinkage cavity tends to occur, while a melt having been filled is in asemi-solid state just before solidification is known as a technology forreducing occurrences of shrinkage cavities during casting (refer to, forexample, Japanese Unexamined Patent Application, Publication No.59-156560 and Japanese Unexamined Patent Application, Publication No.8-155622).

In the technology shown in Japanese Unexamined Patent Application,Publication No. 8-155622, a local pressurization pin is pushed in withan oil hydraulic cylinder just before solidification, the oil hydrauliccylinder is operated until the measured squeeze pressure reaches apredetermined range of the set squeeze pressure while the pressure ofthe local pressurization pin is detected, and the stroke value of thelocal pressurization pin is controlled so as to reduce or eliminate theshrinkage cavity.

However, the technology shown in Japanese Unexamined Patent Application,Publication No. 8-155622 is no more than that the local pressurizationpin is merely pushed in at the time set with a timer while the pressureis detected, and it is not said that the local pressurization pin iscontrolled while the solidification speed and the solidification timingof the melt having been filled in the mold are grasped accurately.

In order to reliably close a solidification shrinkage space which is anopening resulting from the solidification shrinkage, the localpressurization pin must be pressure-controlled while the timing isadjusted in accordance with the solidification. However, in the knownlocal pressurization method, the local pressurization pin control is notperformed while the solidification speed is grasped accurately.Therefore, a probability of success in closing the solidificationshrinkage space is low, and elimination of shrinkage cavity has not yetbeen reached.

As described above, the die-cast pressure vessel is under circumstanceswhere the yield has not reached a satisfactory region against thepressure leakage resulting from the shrinkage cavity.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of suchcircumstances. Accordingly, it is an object of the present invention toprovide a method and an apparatus for manufacturing an aluminum die-castproduct, wherein an occurrence of shrinkage cavity is eliminated orreduced to a very small amount, and a sound and high-quality aluminumdie-cast product having no casting defect can be produced by controllingthe push-in operation of a local pressurization pin.

In order to solve the above-described problems, the method and theapparatus for manufacturing an aluminum die-cast product according tothe present invention adopt the following means.

A method for manufacturing an aluminum die-cast product according to anaspect of the present invention is a method for manufacturing analuminum die-cast product by performing die casting while a part inwhich a casting defect tends to occur due to solidification shrinkage ofa melt, which has been pressure-filled in a mold, during asolidification process is pressurized with a local pressurization pin,the method comprising the steps of allowing the above-described localpressurization pin to follow the movement of a solidification interfacebased on a solidification interface movement speed, which is calculatedin advance, of the above-described melt and keeping pushing the localpressurization pin in until the solidification is completed.

In the method for manufacturing an aluminum die-cast product accordingto an aspect of the present invention, the local pressurization pin isallowed to follow the movement of a solidification interface based on asolidification interface movement speed (depth (mm) from mold surface tolast solidification portion/solidification completion time (s) of lastsolidification portion), which is calculated in advance, of the melt,and the local pressurization pin is kept being pushed in until thesolidification is completed. Therefore, the local pressurization pin canbe pushed in while the timing is adjusted in accordance with thesolidification of the melt. That is, the solidification interfacemovement speed of a portion, which must be locally pressurized, isgrasped accurately by an analysis in advance, and the localpressurization pin is allowed to follow the movement of a solidificationinterface based thereon and is kept being pushed in until thesolidification is completed, so that a solidification shrinkage space(resulting in a shrinkage cavity) can reliably be closed. Therefore, anoccurrence of shrinkage cavity can be eliminated or reduced to a verysmall amount, and a sound aluminum die-cast product having no castingdefect can be produced. Consequently, the yield in the die casting canbe improved significantly, and a high-quality aluminum die-cast productcan be produced at a low cost.

The method for manufacturing an aluminum die-cast product according toan aspect of the present invention may have a configuration in which thepressurization with the above-described local pressurization pin isstarted 0.4 to 1.4 seconds after the filling of the above-described meltis completed.

According to this configuration, since the pressurization with the localpressurization pin is started 0.4 to 1.4 seconds after the filling ofthe melt is completed, the local pressurization pin is allowed to followthe movement of a solidification interface and can reliably be keptbeing pushed in until the solidification is completed. That is, if thestart of pressurization with the local pressurization pin is too early,since solidification is not started and the resistance to pushing-in issmall, the push-in speed becomes too large. Consequently, the localpressurization pin is pushed in up to a stroke end, and it becomesdifficult to keep pushing the local pressurization pin in until thesolidification is completed. Therefore, when the pressurization with thelocal pressurization pin is started within about 1 second from justafter the start of solidification (0.4 seconds after the filling iscompleted), the local pressurization pin is not pushed in up to thestroke end, and the local pressurization pin can reliably be kept beingpushed in until the solidification is completed. In this manner, anoccurrence of shrinkage cavity can be eliminated or reduced to a verysmall amount. If 1.4 seconds have elapsed from the completion offilling, solidification of the part in which the local pressurizationpin is pushed proceeds and it becomes difficult to keep pushing thelocal pressurization pin in until the solidification is completed.

The start of solidification of the melt having been filled is almost thesame regardless of the size and shape of the die-cast product.Consequently, the start time of pressurization with the localpressurization pin can be set as described above, independently of thetype of aluminum die-cast product.

The method for manufacturing an aluminum die-cast product according toan aspect of the present invention may have a configuration in which theaverage push-in speed V of the above-described local pressurization pinis specified to be 1.2 mm/s<V<2.63 mm/s.

According to this configuration, since the average push-in speed V ofthe local pressurization pin is specified to be 1.2 mm/s<V<2.63 mm/s,the local pressurization pin is allowed to follow the movement of asolidification interface and can reliably be kept being pushed in untilthe solidification is completed. In addition, pushing-in can reliably beperformed up to the position suitable for adequately crushing theshrinkage cavity. Here, when the thickness of the part to be locallypressurized is assumed to be about 20 mm, since the solidificationcompletion time of the last solidification portion is 3.8 seconds, thesolidification interface movement speed is 2.63 mm/s. When the localpressurization pin is pushed in while the above-described speed is takenas an upper limit of the pushing-in, the local pressurization pin canreliably be pushed in up to the position suitable for adequatelycrushing the shrinkage cavity. If the push-in speed becomes 1.2 mm/s orless, the resistance to pushing-in becomes excessive, and the localpressurization pin cannot be pushed in. As the depth of pushing-in ofthe local pressurization pin is decreased, the crush of shrinkage cavitytends to become inadequate. Therefore, it is an important factor forreliably closing the solidification shrinkage space (resulting in ashrinkage cavity) to control the push-in speed of the localpressurization pin and ensure an adequate depth of pushing-in. That is,since there is a difference in solidification time from the completionof filling of the melt between the first solidification completionportion (a portion in contact with a mold) and the last solidificationportion, no effect is exerted even when the local pressurization pin ispushed in instantaneously within this period. It is an important factorfor reliably eliminating or reducing an occurrence of shrinkage cavityto push in up to an adequate depth over a few seconds at theabove-described push-in speed.

The method for manufacturing an aluminum die-cast product according toan aspect of the present invention may have a configuration in which theabove-described aluminum die-cast product is a pressure vessel.

According to this configuration, the aluminum die-cast product is apressure vessel. Therefore, a pressure vessel having no casting defect(shrinkage cavity), which becomes a factor of pressure leakage, can beproduced by die casting. Consequently, the yield in the die castingagainst the pressure leakage can be improved significantly, and ahigh-quality pressure vessel can be produced at a low cost.

The method for manufacturing an aluminum die-cast product according toan aspect of the present invention may have a configuration in which theabove-described pressure vessel is a housing of a compressor.

According to this configuration, since the pressure vessel is a housingof a compressor, a highly airtight housing, which does not cause leakageof refrigerant gas sealed in the inside, can be produced by eliminatingthe occurrence of shrinkage cavity or reducing to a very small amount.In particular, with respect to the compressor housing, in many cases, athick-walled boss portion of the housing is worked by cutting aftercasting. Even when the outer surface is clean, if a shrinkage cavity,which is a casting defect, is present in the inside, it is exposed at acutting surface so as to become a factor of the leakage of refrigerantgas. According to the above-described configuration, the occurrence ofshrinkage cavity can be eliminated or reduced to a very small amountand, thereby, a die-cast method most suitable for the compressor housingcan be provided.

An apparatus for manufacturing an aluminum die-cast product according toan aspect of the present invention is an apparatus for manufacturing analuminum die-cast product by performing die casting while a part inwhich a casting defect tends to occur due to solidification shrinkage ofa melt, which has been pressure-filled in a mold, during asolidification process is pressurized with a local pressurization pin,the apparatus including the local pressurization pin disposed inaccordance with the above-described part, in which the casting defecttends to occur, so as to locally pressurize the part, a push-in devicefor pushing the above-described local pressurization pin in toward theabove-described part, and a control portion for controlling theabove-described push-in device in such a way that the above-describedlocal pressurization pin is allowed to follow the movement of asolidification interface based on a solidification interface movementspeed, which is calculated in advance, of the above-described melt andthe local pressurization pin is kept being pushed in until thesolidification is completed.

According to the apparatus for manufacturing an aluminum die-castproduct according to an aspect of the present invention, since thecontrol portion for controlling the push-in device to push in the localpressurization pin in such a way that the local pressurization pin isallowed to follow the movement of a solidification interface based onthe solidification interface movement speed, which is calculated inadvance, of the above-described melt and the local pressurization pin iskept being pushed in until the solidification is completed is included,the local pressurization pin can be pushed in while the timing isadjusted in accordance with the solidification of the melt by thiscontrol portion. That is, the solidification interface movement speed ofa portion, which must be locally pressurized, is grasped accurately bythe analysis in advance, and the local pressurization pin is allowed tofollow the movement of a solidification interface based thereon and canbe kept being pushed in until the solidification is completed, so that asolidification shrinkage space resulting in a shrinkage cavity canreliably be closed. Therefore, an occurrence of shrinkage cavityresulting from the solidification shrinkage can be eliminated or reducedto a very small amount, and a sound die-cast product having no castingdefect can be produced. In addition, the yield in the die casting can beimproved significantly, and a high-quality die-cast product can beproduced at a low cost.

The apparatus for manufacturing an aluminum die-cast product accordingto an aspect of the present invention may have a configuration in whichan overflow portion in accordance with the above-described part, inwhich the casting defect tends to occur, is disposed in theabove-described mold, the above-described local pressurization pin isattached in such a way that the local pressurization pin can be pushedin the above-described overflow portion, and the above-describedoverflow portion is locally pressurized with the above-described localpressurization pin.

According to this configuration, since the overflow portion is disposedin the mold, the local pressurization pin is attached so as to locallypressurize the overflow portion, an occurrence of a trace ofpressurization due to the local pressurization on the die-cast productside can be prevented. Therefore, constraints to the part to be locallypressurized are eliminated, and a higher-quality die-cast product havingno trace of pressurization can be produced.

According to the method and the apparatus for manufacturing an aluminumdie-cast product of the present invention, since the localpressurization pin is allowed to follow the movement of a solidificationinterface of the melt having been filled in the mold and can be keptbeing pushed in until the solidification is completed, so that thesolidification shrinkage space resulting in a shrinkage cavity canreliably be closed. Therefore, an occurrence of shrinkage cavityresulting from the solidification shrinkage can be eliminated or reducedto a very small amount, and a sound aluminum die-cast product having nocasting defect can be produced. In addition, the yield in the diecasting can be improved significantly, and a high-quality aluminumdie-cast product can be produced at a low cost.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a sectional view of an aluminum die-cast product (compressorhousing) produced by a method and an apparatus for manufacturing analuminum die-cast product according to an embodiment of the presentinvention.

FIG. 2 is a partial schematic configuration diagram of an apparatus formanufacturing an aluminum die-cast product according to an embodiment ofthe present invention.

FIG. 3 is an explanatory diagram of a solidification closed loopformation status of a locally pressurized portion, predicted by ananalysis in the method for manufacturing an aluminum die-cast productaccording to an embodiment of the present invention.

FIG. 4 is an explanatory diagram of the quality check results of localpressurization prototypes by a method for manufacturing an aluminumdie-cast product according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment according to the present invention will be described belowwith reference to FIG. 1 to FIG. 4.

FIG. 1 shows an example of an aluminum die-cast product produced by amethod and an apparatus for manufacturing an aluminum die-cast productaccording to the present embodiment. Here, a housing 1 to be used for acompressor is shown as an example of the aluminum die-cast product.

The housing 1 is formed from an aluminum alloy ADC12 (JIS). Acompression mechanism is incorporated in the inside of the housing 1. Arefrigeration gas sealed in a refrigeration cycle is suctioned into thehousing 1, and this is compressed and discharged from the housing 1 tothe outside, so that a function as a pressure vessel is exerted.

The above-described housing 1 takes the shape of a cup having athin-walled cylinder portion 1A, and boss portions 1B having a thicknessa few times larger than the thickness of the cylinder portion 1A areintegrally die-cast at a plurality of positions on the periphery of thecylinder portion 1A. The thickness of the boss portion 1B is specifiedto be 20 mm at the maximum. The boss portion 1B is a portion where ascrew hole, a through hole, or the like is formed by cutting work afterdie casting. If a solidification shrinkage space (resulting in ashrinkage cavity) occurs resulting from solidification shrinkage in theinside of the boss 1B during die casting, the space is exposed at acutting surface by the cutting work. Since this becomes a cause of theleakage of refrigeration gas, elimination of an occurrence of shrinkagecavity during die casting is inevitable in order to produce the housing1 which does not cause pressure leakage nor gas leakage.

FIG. 2 shows a part of a die-cast manufacturing apparatus 10 forproducing an aluminum die-cast housing 1 in which shrinkage cavity thatis a casting defect does not occur.

A mold 11 is mold-clamped with a mold clamping apparatus, although notshown in the drawing, and just after the mold clamping, the inside ofthe cavity 11A is evacuated with a vacuum pump. With respect to the mold11, under the general aluminum die-cast condition, the surfacetemperature of the cavity 11A is controlled at 250° C.±50° C. A melt 12is injected and filled into the cavity 11A of the mold 11 through agate, a plunger, and the like, although not shown in the drawing. Themelt temperature is 680° C.±50° C. under the general aluminum die-castcondition. In general, the injection of the melt 12 is performed by atwo-stage motion of the plunger. In a low speed region, a rod pressureand a head pressure are set at constant pressures, and the plunger ismoved slowly. Switching to a high speed is performed aiming at the timewhen the melt 12 passes the gate. In a high speed region, the plunger isaccelerated sharply, and the filling of the melt 12 into the cavity 11Ais completed in about 0.1 seconds. In general, the speed of thishigh-speed injection is 1.0 m/s or more.

The mold 11 includes an overflow portion 11B connected to the cavity 11.This overflow portion 11B is disposed in accordance with the part inwhich a shrinkage cavity that is a casting defect tends to occur duringcasting. That is, in the above-described housing 1, overflow portionsare disposed at a plurality of positions (merely one position is shownin FIG. 2) in accordance with the bosses 1B (thickness 20 mm) having athickness larger than the thickness of the cylinder portion 1A. A localpressurization apparatus 13, which can independently pressurize the melt12 filled in the above-described individual overflow portions 11B, isattached outside the mold 11.

This local pressurization apparatus 13 is provided with a pressurizationcylinder 14 to be attached to the mold 11 and a local pressurization pin16, which is connected to a piston 15 of the pressurization cylinder 14and which is moved by a predetermined range of stroke. The localpressurization pin 16 is disposed in such a way as to be pushed in theoverflow portion 11B. In the present embodiment, a maximum push-instroke L thereof is set at, for example, 18 mm, and the pin diameter isset at, for example, 9 mm.

The pressurization cylinder 14 is to push the local pressurization pin16 into the overflow portion 11B. As is publicly known, oil hydrauliccircuits 20A and 20B are connected to the pressurization cylinder 14 insuch a way that one chamber 14A is supplied with oil from an oilpressure supply source, although not shown in the drawing, through adirectional control valve 17, a solenoid valve 18, and a throttlingvalve 19, and the oil is discharged from the other chamber 14B throughthe directional control valve 17. The oil hydraulic circuit 20B isprovided with a flowmeter 21 for monitoring the stroke of the localpressurization pin 16.

The directional control valve 17, the solenoid valve 18, the throttlingvalve 19, and the flowmeter 21 disposed in the oil hydraulic circuits20A and 20B of the above-described pressurization cylinder 14 arecontrolled and monitored by a control unit 22.

In order to push the local pressurization pin 16 into the overflowportion 11B while the timing is adjusted in accordance with thesolidification of the melt 12 filled in the cavity 11A of the mold 11,the control unit 22 is configured to be able to control the movement ofthe local pressurization pin 16 by using the following two points asparameters.

(1) The start time ts of the local pressurization pin 16 is determinedby setting a timer for the time of opening or closing of the solenoidvalve 18.

(2) The push-in speed V of the local pressurization pin 16 is controlledby adjusting a flow rate of the pressurizing oil through changing of thevalve opening of the throttling valve 19.

In order to reliably close the solidification shrinkage space andeliminate an occurrence of shrinkage cavity defect by locallypressurizing the part, in which a shrinkage cavity that is a castingdefect tends to occur, with the above-described local pressurizationapparatus 13, it is necessary that the position of occurrence of thedefect is a position suitable for crushing by the pushing-in of thelocal pressurization pin 16 and that the movement of the localpressurization pin 16 is controlled appropriately while the timing isadjusted in accordance with the solidification of the melt 12.

FIG. 3 shows a solidification closed loop formation status of a locallypressurized portion, predicted by an analysis.

A solidification closed loop formation status while the localpressurization pin is in the standby state was predicted. It waspredicted that a solidification closed loop occurred just below thelocal pressurization pin 16 disposed at the maximum thick-walled place(thickness 20 mm), at which a solidification closed loop was expected tooccur, and it was determined that the defect was able to be closed byappropriately controlling the local pressurization pin 16. Furthermore,there was a difference of 3.6 seconds in solidification time, which waspredicted by the analysis, from the completion of filling of the meltbetween the first solidification completion portion (a portion incontact with the mold 11) and the last solidification portion.Therefore, it was estimated that no effect was exerted even when thelocal pressurization pin 16 was pushed in instantaneously within thisperiod, and the local pressurization pin 16 had to be pushed in up to anadequate depth over a few seconds. As also shown in FIG. 3, in thepresent embodiment, solidification of the melt 12 is allowed to start0.2 seconds after the filling is completed, and an elapsed time untilthe solidification is completed is 3.8 seconds.

According to the above-described analysis, it was found that thesolidification shrinkage space (resulting in a shrinkage cavity) wasable to be closed reliably by grasping accurately the solidificationinterface movement speed of a portion, which had to be locallypressurized, in advance, allowing the local pressurization pin 16 tofollow the movement of a solidification interface based thereon, andkeeping pushing the local pressurization pin 16 in until thesolidification was completed. The solidification interface movementspeed of the melt 12 can be calculated based on “depth (mm) from moldsurface to last solidification portion/solidification completion time(s) of last solidification portion”.

According to this finding, local pressurization products were prototypedby using two parameters of the above-described (1) the start time ts ofthe local pressurization pin 16 and (2) the push-in speed V of the localpressurization pin 16, and the quality thereof was checked.

FIG. 4 shows the quality check results of local pressurizationprototypes. The shape (circle, triangle, and rectangle) of the plottedpoint showing each experimental condition indicates the X-ray evaluationresult, the pattern (solid, dots, and empty) of the plotted pointindicates the specific gravity, and the number added to the plottedpoint indicates the average push-in speed V of the local pressurizationpin 16.

As a result, mostly, an improvement is recognized as compared with aknown core pin method (a core pin is inserted by partial pressurizationinto a boss portion 1B in the die casting so as to mold a pilot hole bydie casting). However, as the push-in depth of the local pressurizationpin 16 is decreased, crushing of defect becomes inadequate. As the localpressurization pin is started earlier and the push-in speed V isincreased, the local pressurization pin 16 tends to be pushed in up to astroke end. However, in some cases, minor defects are observed.Therefore, it is believed to be desirable that these conditions areavoided in order to ensure the quality in mass production stably.

The specific gravity in the known core pin method was about 2.67 g/cm³,whereas the specific gravity of each prototype was 2.71 g/cm³ or more.Therefore, an improvement was recognized.

In particular, with respect to all prototypes within the range indicatedby a thick solid line shown in FIG. 4, no defect was recognized in theX-ray evaluation and, in addition, it was recognized that high qualityexhibiting the specific gravity of 2.73 g/cm³ or more was obtained.

That is, the start time of the local pressurization pin 16 is specifiedto be 0.4 to 1.4 seconds after the filling of the melt 12 is completed.It is desirable that the pressurization with the local pressurizationpin 16 is started 0.4 to 1.4 after the filling of the melt 12 iscompleted, and the pushing-in of the local pressurization pin 16 iscontinued until the solidification of the melt 12 is completed (here,3.8 seconds after the filling is completed). If the start ofpressurization is too early, since solidification is not started and theresistance to pushing-in of the melt 12 is small, the push-in speedbecomes too large. Consequently, the local pressurization pin 16 ispushed in up to a stroke end, and it becomes difficult to keep pushingthe local pressurization pin 16 in until the solidification iscompleted. On the other hand, if the start of pressurization with thelocal pressurization pin 16 is too late, since solidification proceeds,the push-in depth of the local pressurization pin 16 is decreased, andcrushing of defect may become inadequate. When the pressurization withthe local pressurization pin 16 is started within about 1 second justafter the filling of the melt 12 is completed and solidification isstarted (0.4 seconds after the filling is completed), the localpressurization pin 16 is not pushed in up to the stroke end, the localpressurization pin 16 can reliably be kept being pushed in until thesolidification is completed, and the local pressurization pin 16 can bepushed in up to an adequate depth.

It is desirable that the average push-in speed V of the localpressurization pin 16 is specified to be within the range of 1.2mm/s<V<2.63 mm/s. That is, when the average push-in speed V of the localpressurization pin 16 is specified to be 1.2 mm/s<V<2.63 mm/s, the localpressurization pin 16 is allowed to follow the movement of asolidification interface of the melt 12 and can reliably be kept beingpushed in until the solidification is completed. In addition, pushing-incan reliably be performed to the position suitable for adequatelycrushing the shrinkage cavity.

Here, when the thickness of the boss portion 1B to be locallypressurized is assumed to be about 20 mm, the solidification completiontime of the last solidification portion is 3.8 seconds (refer to FIG.3). Therefore, the solidification interface movement speed is 10 mm/3.8s, that is, 2.63 mm/s. When the local pressurization pin 16 is pushed inwhile this is taken as an upper limit push-in speed V, the localpressurization pin 16 can reliably be pushed in up to the positionsuitable for adequately crushing the shrinkage cavity. In FIG. 4, thereare conditions, under which no defect is recognized in the X-rayevaluation and high quality exhibiting the specific gravity of 2.73g/cm³ or more is obtained, outside the range indicated by the thicksolid line. However, if the average push-in speed V of the localpressurization pin 16 exceeds 2.63 mm/s, the push-in speed of the localpressurization pin 16 is too large relative to the solidificationinterface movement speed and, thereby, the local pressurization pin 16may be pushed in up to the stroke end before a shrinkage cavity occurs.Therefore, it is desirable that these conditions are avoided as much aspossible in order to ensure high quality in mass production stably.

If the push-in speed becomes 1.2 mm/s or less, the solidificationproceeds and the resistance to pushing-in becomes excessive, so that thelocal pressurization pin 16 may not be pushed in up to an adequatedepth. Therefore, it is also desirable that these conditions are avoidedas much as possible in order to ensure high quality in mass productionstably. If the push-in depth of the local pressurization pin 16 isdecreased, crushing of defect tends to become inadequate. Consequently,it is an important factor for reliably closing the solidificationshrinkage space (resulting in a shrinkage cavity) to control the push-inspeed of the local pressurization pin 16 and ensure an adequate depth ofpushing-in.

According to the present embodiment as described above, the followingoperation and effect are exerted.

According to the present embodiment, the part, in which a casting defect(shrinkage cavity) tends to occur due to solidification shrinkage of themelt 12 pressure-filled in the mold 11 during a solidification process,that is, the boss portion 1B of the compressor housing 1, can bedie-cast by pressurizing with the local pressurization pin 16.

At this time, since the local pressurization pin 16 is allowed to followthe movement of a solidification interface based on the solidificationinterface movement speed, which is calculated in advance, of theabove-described melt and the local pressurization pin 16 is kept beingpushed in until the solidification is completed, the localpressurization pin 16 can be pushed in while the timing is adjusted inaccordance with the solidification of the melt 12. Consequently, thesolidification shrinkage space (resulting in a shrinkage cavity) canreliably be closed and an occurrence of shrinkage cavity can besuppressed in the part in which a shrinkage cavity tends to occur.

Therefore, the occurrence of shrinkage cavity resulting from thesolidification shrinkage can be eliminated or reduced to a very smallamount, and a sound housing 1 having no shrinkage cavity defect can beproduced. In addition, the yield in the die casting can be therebyimproved significantly, and a high-quality housing 1 can be provided ata low cost. Particularly in the compressor housing 1 to be used as apressure vessel, in many cases, a screw hole or a through hole is formedin the thick-walled boss portion 1B, in which a shrinkage cavity tendsto occur, by cutting work after casting. Therefore, even when an outersurface is clean, if a shrinkage cavity is present in the inside, thespace is exposed at a cutting surface and becomes a cause of the leakageof refrigeration gas. However, since the occurrence of shrinkage cavitycan be eliminated or reduced to a very small amount, as described above,a highly airtight housing not causing leakage of refrigerant can beproduced.

Since the pressurization with the local pressurization pin 16 is started0.4 to 1.4 seconds after the filling of the melt is completed, the localpressurization pin 16 is allowed to follow the movement of asolidification interface and can reliably be kept being pushed in untilthe solidification is completed. Therefore, it is avoided that the startof pressurization with the local pressurization pin 16 is too early, thelocal pressurization pin 16 is pushed in up to a stroke end, and itbecomes difficult to keep pushing the local pressurization pin 16 inuntil the solidification is completed. Furthermore, it is avoided thatthe start of pressurization with the local pressurization pin 16 is toolate, the push-in depth of the local pressurization pin 16 is decreased,and the shrinkage cavity defect cannot be crushed adequately.Consequently, a shrinkage cavity can reliably be eliminated or reducedto a very small amount.

Since the average push-in speed V of the local pressurization pin 16 isspecified to be 1.2 mm/s<V<2.63 mm/s, the local pressurization pin 16 isallowed to follow the movement of a solidification interface of the melt12 and can reliably be kept being pushed in until the solidification iscompleted. In addition, pushing-in can reliably be performed up to theposition suitable for adequately crushing the shrinkage cavity.Therefore, the local pressurization pin 16 is allowed to follow themovement of a solidification interface and can be pushed in up to anadequate depth over a few seconds at an appropriate speed. Consequently,the solidification shrinkage space (resulting in a shrinkage cavity) canreliably be closed, so that elimination or reduction of occurrence ofshrinkage cavity can reliably be realized.

In the local pressurization of a part, in which a casting defect(shrinkage cavity) tends to occur, since the overflow portion 11B isdisposed in the mold 11 and the overflow portion 11B is pressurized withthe local pressurization pin 16, an occurrence of a trace ofpressurization due to the local pressurization on the die-cast product,that is, the housing 1, side can be prevented. Therefore, constraints tothe part to be locally pressurized are eliminated, and a higher-qualitydie-cast product having no trace of pressurization can be produced.

In the above-described embodiment, the local pressurization apparatus 10having the configuration, in which the local pressurization pin 16 ispushed in with a pressurization cylinder 14 by using the oil pressure,is explained. The local pressurization apparatus 10 is not limited tosuch an oil hydraulic type. It may be an electric local pressurizationapparatus in which an electric servo motor is used, and the rotationaldrive thereof is converted to a linear motion by a ball screw shaft soas to push in the local pressurization pin 16.

The hosing 1 of the compressor is shown as an example of aluminumdie-cast products, but not limited to this. As a matter of course, thepresent invention may be applied to various aluminum die-cast products,e.g., aluminum alloy cylinders, crankcases, transmission cases, and thelike.

1. A method for manufacturing an aluminum die-cast product by performingdie casting while a part in which a casting defect tends to occur due tosolidification shrinkage of a melt, which has been pressure-filled in amold, during a solidification process is pressurized with a localpressurization pin, the method comprising the steps of: allowing thelocal pressurization pin to follow the movement of a solidificationinterface based on a solidification interface movement speed, which iscalculated in advance, of the melt; and keeping pushing the localpressurization pin in until the solidification is completed.
 2. Themethod for manufacturing an aluminum die-cast product according to claim1, wherein the pressurization with the local pressurization pin isstarted 0.4 to 1.4 seconds after the filling of the melt is completed.3. The method for manufacturing an aluminum die-cast product accordingto claim 1, wherein the average push-in speed V of the localpressurization pin is specified to be 1.2 mm/s<V<2.63 mm/s.
 4. Themethod for manufacturing an aluminum die-cast product according to claim1, wherein the aluminum die-cast product is a pressure vessel.
 5. Themethod for manufacturing an aluminum die-cast product according to claim4, wherein the pressure vessel is a housing of a compressor.
 6. Anapparatus for manufacturing an aluminum die-cast product by performingdie casting while a part, in which a casting defect tends to occur dueto solidification shrinkage of a melt, which has been pressure-filled ina mold, during a solidification process, is pressurized with a localpressurization pin, the apparatus comprising: the local pressurizationpin disposed in accordance with the part, in which the casting defecttends to occur, so as to locally pressurize the part; a push-in devicefor pushing the local pressurization pin in toward the part; and acontrol portion for controlling the push-in device in such a way thatthe local pressurization pin is allowed to follow the movement of asolidification interface based on a solidification interface movementspeed, which is calculated in advance, of the melt and the localpressurization pin is kept being pushed in until the solidification iscompleted.
 7. The apparatus for manufacturing an aluminum die-castproduct according to claim 6, wherein: an overflow portion in accordancewith the part, in which the casting defect tends to occur, is disposedin the mold; the local pressurization pin is attached in such a way thatthe local pressurization pin can be pushed in the overflow portion; andthe overflow portion is locally pressurized with the localpressurization pin.